The present invention relates to compensated crystalline superlattice ferrimagnets. The ferrimagnets include multiple crystalline ferromagnetic layers in combination with crystalline antiferromagnetically coupled layers. The superlattice ferrimagnets may be used in such applications as thermally assisted magnetic data storage disks and the like.
Ferrimagnets are magnetic materials in which the magnetic spins of one class of atoms are opposed to the magnetic spins of another class of atoms within the same material. Compensated ferrimagnets have the additional feature that there exists a temperature for which the spins on the two classes of atoms cancel. An example is the amorphous rare-earth transition metal alloys used in magneto-optical disks. Compensated ferrimagnets have the useful property that their magnetizations vary from zero at the compensation temperature to large values at temperatures remote from the compensation temperature. This leads to large variations in coercivity, which varies inversely with magnetization. These variations can be exploited for various applications such as magneto-optical disks.
Ferrimagnets have been proposed for use as data storage or memory layers in thermally assisted magnetic recording systems. For example, U.S. Pat. No. 5,889,641 to Belser et al., which is incorporated herein by reference, discloses the use of a DyFeCo ferrimagnetic alloy as a data storage layer in a thermally assisted magnetic recording system.
In some applications, crystalline materials are grown directly on the ferrimagnet, e.g., optically assisted magnetic recording (OAMR). This is difficult to accomplish with conventional amorphous ferrimagnet materials currently in use. Furthermore, amorphous ferrimagnetic materials based on rare earth-transition metal alloys are susceptible to environmental corrosion due to the high chemical reactivity of the rare earth components. OAMR and other applications, such as near-field optical recording and thermally assisted magnetic recording systems, require the recording head structure to be positioned in close proximity to the recording layer. This may preclude the use of a thick protective overcoat layer, thus rendering conventional RE-TM alloys susceptible to corrosion.
Although some types of crystalline compensated ferrimagnets such as rare-earth transition metal compounds are known, they exhibit additional problems. First, the crystalline materials are difficult to make with traditional vacuum deposition techniques unless a post-anneal treatment is used, which can destroy many types of substrates. Second, their compensation temperatures are difficult to adjust because the crystalline compounds will typically only form at precise ratios of the constitutive elements. This results in very limited choices of compensation temperatures. Finally, many of the materials have a cubic crystal structure that yields a multi-directional magnetic anisotropy. This can be a disadvantage because uniaxial anisotropy is often desired.
It is known that a superlattice consisting of one layer of Co alternating with two layers of Rh or Ir produces an antiferromagnet (R. H. Victora and J. M. MacLaren, J. Appl. Phys. 70, 5880, 1991). In this superlattice, antiferromagnetic coupling occurs within the Rh or Ir layers and appears to be independent of the ferromagnet material chosen, provided that the ferromagnet material has sufficient magnetic strength to polarize the Rh or Ir layers.
The present invention has been developed in view of the foregoing.
The present invention .provides compensated crystalline superlattice ferrimagnets which include different alternating ferromagnetic layers separated by antiferromagnetically coupled layers. If the ferromagnetic layers have different temperature dependence of the magnetization, then a compensation point may be provided. For example, the structure may consist of the following repeating structure: A/B/B/C/BIB; where A and C are ferromagnetic layers and B represents antiferromagnetically coupled layers. The ferromagnetic A and C layers may comprise metals or alloys such as Ni, Co or Fe, while the antiferromagnetically coupled B layers may comprise metals or alloys such as Rh, Ir, Cr, Ru, Os, W, Mn and the like. Each of the layers need not be a single element but rather may comprise alloys of different metals. The thickness of each B layer may be one monolayer, and the thickness of each A and C layer may be selected to yield a compensation point at the desired temperature. For example, the A layer(s) may be chosen to be a trilayer of Ni, while the C layer(s) may be a monolayer of Co. Fine-tuning may be accomplished by adding a dopant to change Curie temperatures or magnetizations.
The present superlattice structures provide advantages in comparison with other crystalline materials. The superlattice materials can be easily deposited without post-anneal treatments. For example, sputtering techniques using a rotating substrate may be used to control the thickness and dopants of the ferromagnetic layers in order to obtain the desired compensation temperature. Furthermore, the present materials may exhibit uniaxial anisotropy.
An aspect of the present invention is to provide a compensated crystalline superlattice ferrimagnet comprising a first ferromagnetic layer, a second ferromagnetic layer having a different magnetic disordering temperature than the first ferromagnetic layer, and antiferromagnetically coupled layers between the first and second ferromagnetic layers.
Another aspect of the present invention is to provide a method of making a compensated crystalline superlattice ferrimagnet. The method comprises depositing a first ferromagnetic layer on a substrate, depositing antiferromagnetically coupled layers on the first ferromagnetic layer, and depositing a second ferromagnetic layer on the antiferromagnetically coupled layers.
A further aspect of the present invention is to provide a thermally assisted magnetic recording media comprising a substrate, a memory layer deposited on the substrate, and a copy layer deposited on the memory layer. The memory layer . comprises a compensated crystalline superlattice ferrimagnet including a first ferromagnetic layer, a second ferromagnetic layer having a different magnetic disordering temperature than the first ferromagnetic layer, and antiferromagnetically coupled layers between the first and second ferromagnetic layers.
These and other aspects of the present invention will be more apparent from the following description.